IMO...

Good pair of slim-fit jeans (£50 next)
Decent belt (£15 River Island) (and maybe comical belt buckle)
Tight fitting t-shirt (£10 Jack and Jones)
Thin scarf (£5-10 River Island)
Some cool pumps (£20 next)

is a pretty cool combo and cheap as fcuk to piece together. Some key points are:

-Make sure you don't skimp on the jeans (you can always tell if they're cheap)
-Have a nice, but non-generic t-shirt, shirt or t-shirt-cardigan combination
-Accessorise! Wear a cool medallion or scarf; have a cool bracelet and a watch
-Shoes don't have to be too expensive at all; just make sure they compliment your outfit. I've just bought 3 pair of shoes from next for a total of £56 and they suit all occasions and look great!
-If you wear a shirt, have the top button open and have a cool medallion for people to comment on
-If you're up to it, accessorise your face! Couple of piercings wouldn't hurt (well, they might)

It's fairly easy to get your fashion sorted when you look around shops like river island.

Also remember, don't wear pale jeans with a dark shirt!